There are many products of industrial manufacture which require sheet metal stampings with apertures or outline configurations which must be held to close tolerances. A particular example is that of laminations for forming the magnetic circuit cores of dynamoelectric machines, such as stator laminations for motors and generators. In the manufacture of laminated stator cores, the bore of the stator must be kept within close tolerances in order to realize high performance of the rotating machine. The laminations are made from sheet metal having special magnetic properties which is typically supplied in the form of coils of strip stock for high volume production by progressive die stamping. The die stamping operations are carefully controlled during blanking of the stator laminations from the sheet metal strip in order to achieve a high degree of dimensional accuracy. It is especially important to produce laminations with a stator bore which is round, i.e. substantially circular within very close tolerances.
Even though the stamping dies used for forming the laminations are made with a high degree of precision, there are many instances where the stator bores have dimensions outside the desired tolerances. It is not uncommon for the stator bore in an individual lamination to be substantially out-of-round. It has been found that even though the stator bore is of circular shape as it is formed in the sheet metal strip, the stator bore frequently becomes out-of-round after separation from the sheet metal strip. This results in an inferior or unacceptable stator core or in the requirement for costly machining or other special processing to make the bore circular.
The manufacture of stator cores for dynamo electric machines is a prime example of the need for improvement in the production technique for sheet metal stampings, especially as applied to progressive die stamping which require apertures and outlines of precise final dimensions.
Although this need has been known in sheet metal stamping industry for a long time, the prior art has not produced a satisfactory solution.
In the prior art patent to Furth U.S. Pat. No. 2,085,092 granted Jun. 29, 1937, a process is described for fabricating laminations for electromagnetic cores by progressive die stamping wherein the outline of the lamination is substantially fully cut at a first stamping station leaving short uncut portions which are cut at a subsequent station. The invention of this patent is directed to the problem of avoiding strains in the magnetic material which are detrimental to its magnetic properties and utilizes an annealing step between the stamping stations to relieve strains. The Furth patent does not address the problem of achieving precise shapes and dimensions in the finished lamination.
The Sleeter U.S. Pat. No. 2,304,607 granted Dec. 8, 1942 describes a method of making a motor stator which is built up of laminations which are cut out of sheet metal strip by die stamping. In this method, the laminations are stacked on a mandrel to obtain alignment with respect to their internal margins and then they are secured together by rivets. The internal margin is described as being out of alignment or off center due to inaccuracy of the dies which, even though very slight, may result in out-of-round bores. In order to true the stator bore, a burnishing broach is forced through the bore.
In the Gerstle U.S. Pat. No. 3,834,013 granted Sep. 10, 1974, a method is described for forming a finished bore size in laminated stator cores. This patent describes a method in which a heat expandable cylindrical fixture is mounted in an out-of-round bore which is initially formed in a stack of stator laminations. The fixture and the stack are heated to a high temperature so that the fixture is thermally expanded against the sides of the bore to develop radial force which deforms the bore sides into conformity with the expanded circumference of the fixture. This provides a stator bore size having a circular configuration with closely controlled dimensional tolerances.
Also in the prior art is a publication entitled "Lamination Die Design" by G. W. Greenway which was published in the November-December, 1981 issue of the Carbide & Tool Journal. This publication describes certain features of a lamination die for thin metal stamping of great precision in progressive die operations, especially for the manufacture of stator laminations.
The prior art has addressed the problem of achieving dimensional accuracy in finished sheet metal parts only by improved precision in the design and making of the dies and by machining or deforming the margin defining an opening, such as a circular bore, after the stamped part is separated from the sheet metal strip.
An objective of this invention is to provide an improved technique for producing sheet metal stampings with apertures and outlines of precise final dimensions and shapes.